In metal components that are exposed to high temperatures and high stress such as the rotor blades of boiler pipes and gas turbine engines and the like, there is a possibility that defects such as holes (i.e., voids) and cracks and the like will occur due to creep damage or fatigue failure that is caused by age deterioration. Moreover, in metal components which are used in the pipes of reforming plants that create a gas mixture containing hydrogen by reforming natural gas and the like, there is a possibility of defects such as voids and cracks occurring because of hydrogen corrosion. Inspecting the level to which these defects have progressed and accurately predicting the remaining lifespan of metal components is extremely important for planning the timings of inspections or replacements or the like of the relevant metal components.
For example, in Japanese Patent Publication No. 1646031, a technology is described in which ultrasonic waves are irradiated through the surface of a metal component being inspected, and scattered ultrasonic waves that are generated by defects present inside the metal component are detected as noise signals. This makes it possible to quantitatively inspect the level of defect progression. Moreover, because an intimate relationship exists between the level of defect progression and the remaining lifespan of a metal component, it is possible to predict the remaining lifespan of a metal material from the level of defect progression.
Specifically, a ratio (i.e., a spectrum surface area ratio SX/SO) of a surface area value SO of a frequency spectrum that is obtained by performing FFT processing on a noise signal detected when ultrasonic waves are irradiated onto a new metal component having no defects relative to a surface area value SX of a frequency spectrum that is obtained by performing FFT processing on a noise signal detected when ultrasonic waves are irradiated onto a metal component that has been in operation for a predetermined period of time is determined as the level of defect progression.
FIG. 12 (a) shows an example of a noise signal which is detected when ultrasonic waves are irradiated onto a new metal component having no defects. In this drawing, the symbol W1 is irradiated an ultrasonic wave signal, WN is a noise signal, W2 is a bottom surface reflected signal that is obtained when the irradiated ultrasonic wave is reflected by the bottom surface (i.e., the rear surface) of the metal component, and is detected at the front surface thereof. FIG. 12 (b) shows a frequency spectrum which is obtained when the noise signal WN which has been detected in this manner is extracted as a time window corresponding to a time width Tg, and FFT processing is then performed on this extracted signal. FIG. 13 (a) shows an example of the noise signal WN which is detected when ultrasonic waves are irradiated onto a metal component that has been in operation for a predetermined period of time. As is shown in this drawing, when a certain operating time has passed, a large number of defects are generated. As a result, the detected noise signal WN also grows larger. FIG. 13 (b) shows a frequency spectrum which is obtained when the noise signal WN which has been detected in this manner is extracted as a time window corresponding to the time Tg, and FFT processing is then performed on this extracted signal.
Namely, a ratio between the surface area value SO of the frequency spectrum shown in FIG. 12 (b) and the surface area value SX of the frequency spectrum shown in FIG. 13 (b) is the spectrum surface area ratio SX/SO. In contrast, FIG. 14 shows an example of a characteristic curve (referred to below as the remaining lifespan curve) that shows a relationship between a spectrum surface area ratio SX/SO that has been determined by experiment in advance, and a lifespan consumption rate of a metal component. Here, assuming that the remaining lifespan curve relates to creep damage, for example, then the lifespan consumption rate is a ratio of a time t that has elapsed since the metal component began to operate relative to a creep rupture lifespan time tf. If a spectrum surface area ratio SX/SO of 1.5 is obtained for a metal component that has been in operation for a predetermined period of time, then it is found that the lifespan consumption rate of this metal component is approximately 85% based on the aforementioned FIG. 14. Accordingly, the remaining lifespan can be predicted to be 15% of the creep rupture lifespan tf.
Patent document 1: Japanese Patent Publication No. 1646031